Re: [Qemu-devel] [PATCH for-4.1] q35: acpi: do not create dummy MCFG table

[Igor Mammedov <imammedo@redhat.com>]

On Thu, 11 Apr 2019 09:32:11 +0800
Wei Yang <richardw.yang@linux.intel.com> wrote:

> On Wed, Apr 10, 2019 at 05:01:50PM +0200, Igor Mammedov wrote:
> >On Wed, 10 Apr 2019 22:27:56 +0800
> >Wei Yang <richardw.yang@linux.intel.com> wrote:
> >
> >[...]  
> >> >@@ -2411,19 +2410,7 @@ build_mcfg_q35(GArray *table_data, BIOSLinker *linker, AcpiMcfgInfo *info)
> >> >     mcfg->allocation[0].start_bus_number = 0;
> >> >     mcfg->allocation[0].end_bus_number = PCIE_MMCFG_BUS(info->mcfg_size - 1);
> >> > 
> >> >-    /* MCFG is used for ECAM which can be enabled or disabled by guest.    
> >> 
> >> I want to cnfirm what is "enabled or disabled by guest" here.  
> >
> >Firmware theoretically during PCI initialization may disable ECAM support
> >and that's when we do no need MCFG. In practice that's not happening
> >(SeaBIOS or UEFI) but we in case there is out there a firmware that does
> >disable ECAM we do not generate MCFG.
> >
> >Note:
> >ACPI tables generated twice, 1st when QEMU starts and the second time
> >when firmware accesses fwcfg to read blobs for the 1st time.
> >The later happens after PCI subsystem was initialized by firmware.
> >At that time we know if ECAM was enabled or not.
> >  
> 
> That's much clear, thanks :-)
> 
> So this is the guest BIOS instead of guest kernel who may disable/enable it.
> 
> >> If we don't reserve mcfg and "guest" enable mcfg during running, the ACPI
> >> table size changed. But the destination still has the original table size,
> >> since destination "guest" keep sleep during this period.
> >> 
> >> Now the migration would face table size difference  
> >
> >with commit a1666142db we do not care as all the tables created on
> >source will be migrated to destination as is overwriting whatever blobs
> >destination created on startup.
> >  
> >> and break migration?  
> >nope,
> >
> >to help you figure out why it works
> >look at what following git commits did:
> >  git log c8d6f66ae7..a1666142db
> >and pay attention to 'used_length'
> >  
> 
> To be honest, this is what I feel confused in your previous reply.
> 
> First I want to confirm both fields in RAMBlock affects the migration:
> 
> * used_length
> * max_length
> 
> Both of them should be the same on both source/destination, otherwise the
> migration would fail.
well, it works fine for me.
Where do you see max_length being used during migration?


> Then I thought the migration would be broken if source/destination has
> different knowledge about acpi table size. Because this will introduce
> different value of used_length, even we have resizable MemoryRegion.
> 
> The 1st time ACPI generation flow:
> 
>     acpi_add_rom_blob
>         rom_add_blob
>             rom_set_mr
>                 memory_region_init_resizable_ram
>                     qemu_ram_alloc_resizable
>     		        new_block->used_length = size
>     		        new_block->max_length = max_size
> 
> The 2nd time ACPI generation flow:
> 
>     acpi_ram_update
>         memory_regioin_ram_resize
>             qemu_ram_resize
>                 block->used_length = new_size
> 
> The max_length is always the same, while used_length would be changed to the
> actual table_blob size.
> 
> In case source/destination has different knowledge about acpi table size, the
> table_blob size(even after aligned) could be different.
> 
> This is why I thought there is still some chance to break migration after
> resizable MemoryRegion.
> 
> Do I miss something?
yes, you did, max_length does not influence migration stream.
see what above mentioned commits and ram_load() -> "if (length != block->used_length)" do.